1. Field of the Invention
This invention relates to a two-component type developer employing a magnetic carrier, used to develop electrostatic images used to develop electrostatic images in electrophotography, electrostatic recording and so forth. It also relates to an image forming method.
2. Related Background Art
As electrophotography, various methods are disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth. In these methods, copies or prints are obtained by forming an electrostatic latent image on a photosensitive layer of an electrostatic image bearing member upon irradiation of a light image to form an electrostatic image, subsequently causing a toner to be attracted onto the electrostatic image to develop it to form a toner image, and transferring the toner image to a transfer medium such as paper as occasion calls, followed by fixing by heat, pressure, heat and pressure, or solvent vapor.
In the step of developing the electrostatic image, the toner image is formed by utilizing an electrostatic mutual action between a toner triboelectrically charged and the electrostatic image. Among methods of developing electrostatic images by the use of toners, a developing method making use of a two-component type developer formed of a blend of toner and carrier is commonly preferably used in full-color copying machines or printers which are required to form high-quality images.
In such a developing method, the carrier imparts positive or negative electric charge to the toner in an appropriate quantity by triboelectric charging, and carries the toner on its surface by electrostatic attraction attributable to the triboelectric charging.
The developer having the toner and the carrier is coated on a developing sleeve internally provided with a magnet, in a prescribed layer thickness by means of a developer layer thickness regulation member, and then transported, by utilizing a magnetic force, to a developing zone formed between the electrostatic image bearing member (photosensitive member) and the developing sleeve.
A certain development bias voltage is kept applied across the photosensitive member and the developing sleeve, and the toner participates in development on the photosensitive member In the developing zone.
There are various performances required for the carrier. Especially important performances may include appropriate charging performance, breakdown strength to applied voltage, impact resistance, wear resistance, spent resistance and development contribution.
For example, when developers are used for a long period, a toner called a spent-toner, which does not contribute to the development, may melt-adhere to the carrier surface to cause toner filming, so that this causes a deterioration of the developer and concurrently with it a deterioration of image quality of developed images.
In general, a carrier having too large a true specific gravity may apply a great load on the developer when the developer is formed on the developing sleeve in a prescribed layer thickness by means of the developer layer thickness or when the developer is agitated in a developing assembly. Thus, such a carrier may cause (a) toner filming, (b) carrier break and (c) toner deterioration. As the result, this tends to cause the deterioration of developer and concurrently with it the deterioration of image quality of developed images.
With an increase in particle diameter of carriers, the load applied to developer increases like the above, and hence the above (a) to (c) tend to occur, so that the deterioration of developer tends to occur. Also, (d) fine-line reproducibility In the developed images tends to lower.
Accordingly, carriers tending to cause the above (a) to (c) make it necessary to take time and labor to change developers for new ones periodically. Also, since such carriers are uneconomical, it is desirable to lessen the load applied to developers or to improve the impact resistance and spent resistance of carriers so as to prevent the above (a) to (c) and elongate the service life of developers.
Making the carrier have a smaller particle diameter makes (e) the carrier tend to adhere to the electrostatic image bearing member. Also, in an instance where the toner has a constant particle diameter and only the carrier is made to have a small particle diameter, (f) the toner has a broader charge quantity distribution to tend to cause a phenomenon that a toner having caused charge-up jumps unwantedly to non-image areas (hereinafter called "fog") especially when developed in an environment of low humidity.
As a carrier to solve the above problems (a) to (f), a magnetic-fine-particle-dispersed resin carrier is known in the art. This carrier has particles having less shape-originating strain, can relatively easily be made spherical, giving a high particle strength, and has a good fluidity. It also enables wide-range control of particle size distribution. Hence, this carrier is suited for high-speed copying machines or high-speed laser beam printers in which the developing sleeve or the magnet in the sleeve is rotated at a large number of revolutions.
The magnetic-fine-particle-dispersed resin carrier is disclosed in Japanese Patent Applications Laid-open No. 54-66134 and No. 61-9659. However, such a magnetic-particle carrier has a small saturation magnetization unless a magnetic material is incorporated in a large quantity. This tends to cause the carrier to adhere to the electrostatic image bearing member at the time of development, and may make it necessary to replenish the developer or to internally provide an image forming apparatus with a mechanism for collecting the carrier having adhered.
In the case when the magnetic material is incorporated in a large quantity in the magnetic-fine-particle-dispersed resin carrier, the magnetic material is-large in quantity with respect to the binder resin, resulting in a weak impact resistance. Thus, when the developer is formed on the developing sleeve in a prescribed layer thickness by means of the developer layer thickness regulation member, the magnetic material tends to come off the carrier, consequently tending to cause the deterioration of developer.
In addition, in the case when the magnetic material is incorporated in a large quantity in the magnetic-fine-particle-dispersed resin carrier, a magnetic material having a low resistivity is in large quantity to make the carrier have a low resistivity. As the result, faulty images tend to occur because of a leak of bias voltage applied at the time of development.
A technique to coat carrier cores with a resin is disclosed in Japanese Patent Application Laid-open No. 58-21750. Such a resin-coated carrier can be improved in spent resistance, impact resistance and breakdown strength to applied voltage. Also, on account of charging properties of the resin for coating, the charging performance of the toner can be controlled. Accordingly, the desired electric charges can be imparted to the toner by selecting resins for coating.
However, even in the resin-coated carrier, when the resin is coated in a large quantity and the resistivity of carrier is high, the phenomenon of charge-up of toner tends to occur in an environment of low humidity. Also, when the resin is coated in a small quantity, the carrier may have so excessively low a resistivity that faulty images tend to occur because of a leak of bias voltage.
Even when the resistivity of resin-coated carrier is judged to be a proper resistivity on measurement, some coating resins tend to cause faulty images because of a leak of development bias voltage or tend to cause the phenomenon of charge-up of toner in an environment of low humidity.
As a carrier improved in surface contamination resistance, impact resistance, environmental dependence of charging, rise of charging, exchange performance of electric charges and so forth, Japanese Patent Application Laid-open No 4-198946 discloses a magnetic carrier comprising magnetic core particles surface-treated with an aminosilane coupling agent and having coat layers formed of a resin having functional groups capable of reacting with it. Japanese Patent Applications Laid-open No. 7-10452, No. 10-39547 and No. 10-39549 (U.S. Pat. No. 5,766,814) disclose a magnetic carrier provided with silicone resin coat layers containing a silane coupling agent. However, in the carriers disclosed in the above publications, it is difficult to control the reactivity of the silane coupling agent. As the result, charge characteristics tend to vary under the influence of residual functional groups and unreacted matter and also the resistivity can be controlled with difficulty. Thus, there remains a problem for imparting stably to the toner a sufficient charging performance having less environmental variations. In developers also proposed, the coat resin adheres in such an insufficient strength that the coat resin tends to come off when large-area images involving a large toner consumption are copied on a large number of sheets, tending to cause changes in charge quantity of toners.
Thus, it is sought to provide a magnetic carrier that can meet severe requirements nowadays made on quality, e.g., can be adapted to various copying objects such as fine lines, small characters, photographs and color originals and also can satisfy the achievement of high image quality, high grade, high speed and high running performance.